In UV-curable ink for lithography, flexography, and silk screening, solder resist, etching resist for production of industrial electronic material or the like, color filter resist, UV powder coating, aqueous UV coating, coating with UV-absorber, wood article finish coating, plastics, metal coating, etc., there has been employed a photocurable composition containing a radical-polymerizable compound (i.e., a radical-polymerizable monomer) and a radical polymerization initiator, which generates radicals through irradiation with light such as a UV ray for initiating polymerization of the radical-polymerizable compound. A typical radical polymerization initiator is an acetophenone-type radical polymerization initiator (see Patent Documents 1 and 2).
One known acetophenone-type radical polymerization initiator is 2,2-dimethoxy-1,2-diphenylethan-1-one. The compound can generate radicals via self-cleavage through irradiation with light such as a UV ray, to thereby initiate polymerization of a radical-polymerizable compound, and is commercially available as IRGACURE 651 (product of BASF). As compared with other acetophenone-type radical polymerization initiators; e.g., 1-hydroxycyclohexyl phenyl ketone (IRGACURE 184, product of BASF) and 2-hydroxy-1-{4-[4-{2-hydroxy-2-methylpropionyl}-benzyl]-phenyl}-2-methylpropan-1-one (IRGACURE 127, product of BASF), 2,2-dimethoxy-1,2-diphenylethan-1-one has higher absorbance to 365 nm light. This property is advantageous, for the compound can effectively utilize i-line (wavelength: 365 nm), which is a predominant component of high-pressure mercury lamps and other industrially useful light sources.
Differing from thermal polymerization, photopolymerization by use of such a radical polymerization initiator can provide a cured product through irradiation with light such as a UV ray at room temperature or thereabout without heating. Through photopolymerization, the risk of material deterioration is reduced, and a cured product can be obtained in a considerably shorter period of time as compared with thermal polymerization. Thus, photopolymerization is a useful technique widely employed in the industry. However, photopolymerization has a drawback in that a component originating from a radical polymerization initiator remaining in the cured product bleeds out with lapse of time. The bleeding out increases considerably when the amount of radical polymerization initiator is increased for shortening curing time.
Compounds such as 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one (IRGACURE 2959, product of BASF) and 2-hydroxy-1-{4-[4-{2-hydroxy-2-methylpropionyl}-benzyl]-phenyl}-2-methylpropan-1-one (IRGACURE 127, product of BASF) are low-volatile compounds, and give off less odor after curing. These compounds are resistant to bleeding out, but the resistance is not satisfactory.
Hitherto, a technique of reducing bleeding out has been disclosed (see Patent Documents 3 and 4). The technique employs species of (meth)acryloyloxy group-incorporated 2,2-dimethoxy-1,2-diphenylethan-1-one.
However, the compounds disclosed in Patent Documents 3 and 4 have low crystallinity and assume the form of oil. Therefore, in production of these compounds, crude products must be purified through distillation, column chromatography, or a like technique, making the production steps cumbersome. Particularly, since the ketal structure and the (meth)acryloyloxy groups of 2,2-dimethoxy-1,2-diphenylethan-1-one have poor thermal stability, these moieties may undergo thermal decomposition or thermal polymerization during large-scale purification by distillation at high temperature. Also, performing column chromatography disadvantageously elevates production cost.